This is probably likely to have happened when re-naming your mixes in iTunes. The reason it has turned into a .txt formatted file is that when you rename a file you need to make sure to include the original extension it was.
When recording, the standard extensions will be .WAV and .m4a (AAC). This is because of the way you record in your app.
If this is taken off the end of the file name when changing the name, you'll end up with .txt. To avoid this, when changing the name, make sure you add the relevant extension at the end.
For example: evermixrecording.m4a
IMPORTANT: We do advise before re-naming a file is that you save it to your laptop first then re-name as you can potentially lose the file on you Evermix DJ App.